ARROW-359: Document ARROW_LIBHDFS_DIR

Author: Christopher C. Aycock <christopher.aycock@twosigma.com>

Closes #196 from chrisaycock/ARROW-359 and squashes the following commits:

52ec78e [Christopher C. Aycock] ARROW-359: Document ARROW_LIBHDFS_DIR

Author: Christopher C. Aycock

cpp/doc/HDFS.md

@@ -33,16 +33,18 @@ interface to the Java Hadoop client. This library is loaded **at runtime**
 (rather than at link / library load time, since the library may not be in your
 LD_LIBRARY_PATH), and relies on some environment variables.
 
-* `HADOOP_HOME`: the root of your installed Hadoop distribution. Check in the
-  `lib/native` directory to look for `libhdfs.so` if you have any questions
-  about which directory you're after.
-* `JAVA_HOME`: the location of your Java SDK installation
+* `HADOOP_HOME`: the root of your installed Hadoop distribution. Often has
+`lib/native/libhdfs.so`.
+* `JAVA_HOME`: the location of your Java SDK installation.
 * `CLASSPATH`: must contain the Hadoop jars. You can set these using:
 
 ```shell
 export CLASSPATH=`$HADOOP_HOME/bin/hadoop classpath --glob`
 ```
 
+* `ARROW_LIBHDFS_DIR` (optional): explicit location of `libhdfs.so` if it is
+installed somewhere other than `$HADOOP_HOME/lib/native`.
+
 ### Mac Specifics
 
 The installed location of Java on OS X can vary, however the following snippet